Bug: Edge case crashes

[dshkol]

Summary

Three low-frequency but real crash scenarios:

H8: pull(date_field) without data context when sample_date is NA

• File: R/cansim.R:93 (approximate line)
• Issue: Falls back to pull(date_field) without a data context
• Impact: Can error on edge cases

H9: view_cansim_webpage NULL check after lowercase

• File: R/cansim.R:923 (approximate line)
• Issue: Lowercases input before checking for NULL, errors on length-0 input
• Impact: Edge case crash

M8: Potential NULL dereference in cache check

• File: R/cansim_parquet.R:302 (approximate line)
• Issue: is.na(last_downloaded) doesn't handle NULL/empty vector
• Impact: Crash on unexpected cache state

Proposed Fix

• H8: Add proper data context
• H9: Check NULL/length before lowercase
• M8: Check is.null() || length() == 0 || is.na()

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From code audit - low-frequency but real crash scenarios